Coal, formed from decomposed and compressed vegetable matter, is typically found in substantially horizontal seams extending between sedimentary rock strata such as limestone, sandstone or shale. Surface and underground mining are the primary techniques used to recover this coal.
Surface or strip mining involves the removal of material, known as overburden, overlying a coal seam so as to expose the coal for recovery. In recent years, surface mining has gained prominence over underground mining in the United States. This is due to many factors including:
(a) the increased material moving capacity of surface or strip mining equipment; PA1 (b) lower costs for surface mining than underground mining; PA1 (c) the better safety record of surface mining versus underground mining; PA1 (d) the higher coal recovery percentage for surface mining versus underground mining; and PA1 (e) the fact that geological factors favor extraction of many coal reserves by surface mining.
Surface mining does, however, have its limitations despite the advantages cited above. The primary limiting factor relates to the depth of the overburden. Once the coal seam reaches a certain depth below the surface, the amount of overburden that must be removed to reach the coal simply makes strip mining economically unfeasible.
When this occurs, large quantities of coal may still remain in the ground. Other mining methods must, however, be utilized if economic recovery of this coal is to be achieved. Underground mining application in such an instance is, typically, very limited. This may be due to a number of factors including the existence of poor roof support conditions, the thinness of the seam and/or the presence of insufficient quantities of coal to warrant the large capital investments characteristic of underground operations.
Due to these considerations, auger mining is often used to recover coal following a strip mining operation where the overburden becomes too costly to remove. A large auger is used to bore into the face of the seam and recover the coal from beneath the overburden. Advantageously, auger mining is very efficient providing more tons per man per day than any other form of mining. Auger mining may also be initiated quickly and requires a relatively low capital expenditure when compared to surface and underground mining. Auger mining has also been found to date to be the best method to use in relatively thin seams. Further, auger mining is safer than both surface and underground mining. Thus, auger mining may be used to effectively supplement a strip mining operation and recover small coal deposits that would otherwise be left behind.
Auger mining is, however, also not without its disadvantages. Auger mining provides a relatively low total coal recovery. Coal recovery for the resource area being augured is usually less than about 35%. Some of the lost recovery is due to the pillars of coal that are left standing to support the overburden between adjacent auger holes. The majority of the recovery shortfall, however, is due to the limited penetration depths achievable with auger mining equipment.
More particularly, as penetration depths increase, a greater number of auger flights are required to convey the coal from the cutting head to the seam face for recovery. Each flight adds to the frictional resistance to the turning of the auger through contact with the walls of the bore hole. Additionally, the longer the string of auger flights, the greater the weight of coal being moved by the flights at any one time. As a result, it should be appreciated that auger power requirements increase rapidly with the depth of auger penetration.
Due to the above considerations, holes drilled by conventional auguring equipment are usually only of a depth of 150' with 200' being rarely attainable. Of course, any increase in this figure is desirable as it would greatly improve the coal recovery rate from a resource area.